This invention relates to an apparatus for exposing a photosensitive layer through a photomask carrying microscopic patterns, such as of an integrated circuit, and more particularly to a light exposure apparatus suitable for use in photolithographically printing a micro-circuit on a semiconductor substrate.
The exposure of a photosensitive layer such as a photoresist layer applied on a substrate is associated with two objectionable problems of damaging the printed image, one of which is caused by the diffraction of light, and another of which is due to the standing wave occurring in the photosensitive layer.
Conventionally, it has been known that negative films carrying microscopic patterns, when printed by the contact-printing method, provide no perfectly reproduced patterns because of the influence of diffraction which appears in a phenomenon as arising from the insufficient contact between the negative film and the photosensitive layer. This phenomenon is serious in the non-contact printing method which has been developed to prevent damage of the photomask as well as of the wafer which will be caused each time when the photomask is brought into intimate contact with the photosensitive layer applied on the wafer. An apparatus which has overcome this drawback by utilizing the physical phenomenon that one diffraction pattern can be cancelled by superposition of another diffraction pattern known in German laid open for inspection application 2,140,549.
On the other hand, when the light flux impinging upon the wafer is of a monochromatic light, the standing (stationary) waves are caused to appear in the photosensitive layer applied on the wafer surface, resulting in formation of wrinkles on the hardened portions of the developed and processed photosensitive layer of the wafer. In order to overcome this drawback, it is desirable to provide a light-permeable layer of a specified thickness and having an index of refraction almost equal to that of the photosensitive layer located between the photosensitive layer and the wafer and to expose this photosensitive layer to a light flux containing many different wavelength rays.